Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative-working. A positive photographic image can be formed by producing a negative photographic image and then forming a second photographic image which is a negative of the first negative, that is, a positive image. A direct-positive image is understood in photography to be a positive image that is formed without first forming a negative image. Positive dye images which are not direct-positive images are commonly produced in color photography by reversal processing in which a negative silver image is formed and a complementary positive dye image is then formed in the same photographic element. The term "direct reversal" has been applied to direct-positive photographic elements and processing which produces a positive dye image without forming a negative silver image. Direct-positive photography in general and direct reversal photography in particular are advantageous in providing a more straight-forward approach to obtaining positive photographic images.
One conventional approach for obtaining direct-positive photographic images is to employ silver halide emulsions which are initially surface fogged. Surface fog is imagewise removed in exposed areas. When a photon is absorbed by a silver halide grain, a hole (a positive charge) and an electron are released in the silver halide crystal. Migration of the hole to the grain surface results in oxidation of a metallic silver atom (fog) to its ionic form, thereby reducing the developability of the grain.
Surface fogged direct-positive emulsions can be sensitized by incorporating compounds capable of accepting the photon generated electrons, thereby reducing recombination of photon generated hole-electron pairs and rendering more holes available for bleaching surface fog. Compounds which are electron acceptors, thereby sensitizing fogged direct-positive emulsions, are sometimes referred to as desensitizers, since these compounds desensitize negative-working and other silver halide emulsions which rely on photogenerated electrons to form latent image sites. Shiba et al U.S. Pat. No. 3,933,505 employs nitro-substituted fluorenes for this purpose.
An entirely different approach to forming direct-positive images, to which the present invention relates, is to use photographic elements employing internal latent image-forming silver halide grains. After imagewise exposure, the silver halide grains are developed with a surface developer, that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a negative-working photographic element. The internal latent image-forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a comparatively slow rate, as compared to the internal latent image-forming silver halide grains not imagewise exposed. The result is a direct-positive silver image. In color photography, the oxidized developer that is produced during silver development is used to produce a corresponding positive, direct reversal dye image. Multicolor direct reversal photographic images have been extensively investigated in connection with image-transfer photography.
It has been found advantageous to employ nucleating agents in preference to uniform light exposure in the process described above. The term "nucleating agent" is employed herein to mean a fogging agent capable of permitting the selective development of internal latent image-forming silver halide grains which have not been imagewise exposed in preference to the development of silver halide grains having an internal latent image formed by imagewise exposure.
While nucleating agents have been long known to the photographic art, recent interest has focused on identifying nucleating agents that are effective in relatively low concentration levels and that can be incorporated directly into silver halide emulsions. Exemplary of known incorporated nucleating agents are those disclosed by Whitmore U.S. Pat. No. 3,227,552, Lincoln et al U.S. Pat. No. 3,615,615, Spence et al U.S. Pat. No. 3,718,470, Kurtz et al U.S. Pat. Nos. 3,719,494 and 3,734,738, Lincoln et al U.S. Pat. No. 3,759,901, Leone et al U.S. Pat. Nos. 4,030,925 and 4,080,207, Adachi et al U.S. Pat. No. 4,115,122, von Konig et al U.S. Pat. No. 4,139,387, and U.K. Pat. Nos. 2,011,391 and 2,012,443.